1. Field of the Invention
The present invention relates to zoom lens systems which are suitable, for example, for use as a photographing optical system for digital still cameras.
2. Description of the Related Art
Digital still cameras for capturing still images using a solid-state image sensor, such as a CCD (charge-coupled device) sensor or a CMOS (complementary metal oxide semiconductor) sensor, have received attention in recent years. It is desirable to have such digital still cameras small and compact in size. Accordingly, it is desirable for a photographing optical system serving as one of the units of a digital still camera to be very short in total length.
In addition, it is desirable for the photographing optical system to have a wide angle of field in consideration of capturing still images. Moreover, it is desirable for the photographing optical system to provide high optical performance since the digital still camera uses a solid-state image sensor having a greater number of pixels than that of a video camera designed to capture moving images.
Many types of fast zoom lenses having a wide angle of field and providing high optical performance while having a relatively low zoom magnification of 2.5× through 3× or thereabout have been proposed. For example, Japanese Patent Publication No. 6-66008 (corresponding to U.S. Pat. No. 4,662,723) discloses a zoom lens having two lens units of negative and positive refractive powers, respectively, in order from the object side to the image side. Zooming is effected by varying the space between the two lens units.
Japanese Patent Publication No. 7-52256 (corresponding to U.S. Pat. No. 4,733,952) discloses a zoom lens having three lens units of negative, positive and positive refractive powers, respectively, in order from the object side to the image side. The space between the second lens unit and the third lens unit increases during zooming from the wide-angle end to the telephoto end.
U.S. Pat. No. 5,434,710 discloses a zoom lens having three lens units of negative, positive and positive refractive powers, respectively, in order from the object side to the image side. The space between the second lens unit and the third lens unit decreases during zooming from the wide-angle end to the telephoto end.
Japanese Laid-open Patent Application No. 60-31110 discloses a zoom lens having four lens units of negative, positive, positive and positive refractive powers, respectively, in order from the object side to the image side. The space between the second lens unit and the third lens unit decreases during zooming from the wide-angle end to the telephoto end, and the fourth lens unit is stationary during zooming.
U.S. Pat. Nos. 4,477,153, 5,321,553, 5,715,097, 5,721,642, 5,847,875, 5,872,660, 5,969,878, 6,038,084 and 6,169,635 and Japanese Laid-Open Patent Application Nos. 2-118509, 10-213745 and 2001-100098 disclose a zoom lens having three lens units of negative, positive and positive refractive powers, respectively, in order from the object side to the image side. The first lens unit includes three or more lens elements and has one or more aspheric surfaces.
Japanese Laid-Open Patent Application No. 2001-42218 (corresponding to U.S. Pat. No. 6,304,389) discloses a zoom lens having an angle of field of 70° or more at the wide-angle end and a zoom ratio of about 3.6.
However, the zoom lens disclosed in Japanese Patent Publication No. 6-66008 has a great number of lens elements and is, therefore, disadvantageous in terms of compactness. In addition, the position of en exit pupil varies to a great extent during zooming. Therefore, although the zoom lens can be used as a photographing optical system of a silver-halide film camera without problems, the size of the zoom lens tends to increase because design limitations are imposed on power arrangement when the zoom lens is used as a photographing optical system of a digital still camera or the like using a solid-state image sensor.
In the zoom lens disclosed in Japanese Patent Publication No. 7-52256, the position of en exit pupil varies to a great extent during zooming, as in the zoom lens disclosed in Japanese Patent Publication No. 6-66008.
The zoom lenses disclosed in U.S. Pat. No. 5,434,710 and Japanese Laid-open Patent Application No. 60-31110 have a great number of lens elements and, therefore, have limitations on compactness.
The zoom lenses disclosed in U.S. Pat. Nos. 4,477,153, 5,321,553, 5,715,097, 5,721,642, 5,847,875, 5,872,660, 5,969,878, 6,038,084 and 6,169,635 and Japanese Laid-Open Patent Application Nos. 2-118509, 10-213745 and 2001-100098 are insufficient to attain a reduction in size while securing an angle of field at the wide angle end and a zoom ratio that are greater than respective predetermined values.
In the zoom lens disclosed in Japanese Laid-Open Patent Application No. 2001-42218, it is necessary for the second lens unit to have a great number of lens elements so as to perform aberration correction. Therefore, sufficient reduction in size of the whole lens system cannot be attained.